Prince William County, Manassas and Manassas Park have told the State Water Control Board in a formal agreement that they will repair leaks in storm drains, culverts and manholes that have allowed storm water to flood the area's sewer system.

The flooding caused backups at the Upper Occoquan Sewer Authority treatment plant in Centreville and for a short time decreased the quality of the treatment, according to Jeanie Grandstaff, the board's enforcement specialist.

The problems began in February, March and April when heavy rains, combined with melting snow, leaked into the authority's sewer lines.

Although Fairfax County shares sewer capacity with the other three jurisdictions, it does not have the leakage problem.

According to Grandstaff, Fairfax has a continuing repair program that has prevented storm water leakage into sewer lines.

Because the sewer authority has some of the strictest water treatment standards in the country, the small amount of quality loss during the storm water infiltration into the sewer lines did not present a health hazard, Grandstaff said.

The authorty's board approved a $27 million expansion of the$82 million plant early in the summer.

Grandstaff said that a consultant to the authority told the water control board that the expansion would not be necessary if it were not for the leaking sewer lines.

The authority's plant has a15 million-gallon-a-day capacity, which will increase to 22 million gallons a day when the expansion is completed in about four years.

Although the three jursidictions have spent $2 million in federal funds to repair the leaks, each has pledged thousands more to finish the job.

Prince William County is expected to spend $1.2 milion in 1985 and $700,000 in 1986. Manassas plans to spend $550,000, and Manassas Park, which has spent $500,000, will add $200,000 to that in 1985.

Each jurisdiction has submitted "impressive" repair schedules to the water board, said Grandstaff.

The schedules show that some types of repairs will be finished before the time they were originally slated for completion.

The three jurisdictions are to present quarterly reports on their repair programs, as well as reports on the number of new construction starts, Grandstaff said.

Although the agreement signed by the three jurisdictions states that the board could temporarily halt new construction in 1986 if it believes that repairs are not going fast enough to handle additional burdens on the system, "it's unlikely that we'll ever have to enforce that clause," Grandstaff said.